Re: Strange situation, any ideas?
Mostly, I would say changes in wind. Spending alot of time practising for F class shooting, you can really see the affect wind, light/mirage and THERMALS have on POI. I have had groups move 2MOA at 300m just on condition change.
The 308 gets moved around alot so doesn't take much of a change for a big change in POI at LR. A let up in head wind WILL affect POI.
I would suggest a repeat of the shooting with the 'new' reload and see what you get. Calm conditions with overcast skies, early in the morning would be ideal.
You can get thermals anytime of year and with the terrain you shoot over, I would expect them to be quite common and very large. A small bubble can be 100yds across (some only a few ft) and last for a seconds to minute or two. Some bubbles can be 1/2 mile across.
If there is a glider club or private airfield in your area, talk to them about the weather patterns/ground effects. The upward velocity of these thermals can be very impressive. Think moving a Boeing 747 a few ft or more (called air turbulence) while it is traveling 450mph. Or making commercial planes crash on take off/landing (commonly refered to as wind shear).
Unfortunately, we have no way to watch for this occurance (no vertical wind flags, at least not yet).
Something to add to the mix of confusion.